1. Field of the Invention
This invention relates to telephone circuits and, more particularly, to noise filter apparatus for telephone lines.
2. Description of the Prior Art
From the beginning of telephonic systems, noise has been a problem. The term "noise" refers to any type of undesirable signals on a telephone line which interferes with communication or communication signals on the lines.
With the increased use of modems for communicating between computers using telephone lines and the use of Facsimile transmission machines also using telephone lines, the problem of noise on telephone lines becomes increasingly acute. This is so even though telephone lines inherently, due to advances in telephone systems and associated circuitry, technology, etc., are more and more noise free. However, there is still noise or undesirable signals at various frequencies in telephone lines, and such noise may interfere with, and cause problems with, facsimile machines and computer modems by distorting the appropriate signals or adding to the signals to distort the actual signals being transmitted and received.
Typically, echoes are used by modems and by facsimile transmission equipment between transmitting and receiving equipment to verify original data. However, the echoes tend to reverberate and eventually turn into or become noise on the line. The undesirable echoes need to be filtered out of the circuitry before they become noise and interfere with proper and desired transmission.
An early patent, U.S. Pat. No. 308,019 (Van Hoevenbergh) discloses an early attempt to reduce noise by using a condenser in the line and connected to ground or earth.
U.S. Pat. No. 781,625 (Stone) discloses another use of capacitors to reduce noise. In one embodiment, a pair of capacitors is connected between the telephone lines, and the connection between the capacitors is grounded.
U.S. Pat. No. 1,791,377 (Proskauer et al) discloses another type of filter system for reducing alternating current hum in radios, and the like. The apparatus includes a pair of condensers coupled between the power line between the power outlet and the radio set. The junction of the capacitors is grounded. In the two line conductors themselves are resistors. That is, a resistor is in each conductor between the outlet and the capacitors.
U.S. Pat. No. 1,824,819 (Houck) discloses another type of electrical filter system for filtering radio frequency from power lines utilizing capacitors and inductors.
U.S. Pat. No. 1,949,996 (Fleming) discloses another system of connecting electrical appliances to a power line to filter out ac hum and other undesirable interference from radios connected to the power line. Again, capacitors are used. One embodiment discloses resistors in each of the power conductors with a pair of capacitors coupled between the conductors and the coupling between the capacitors is grounded. Other embodiments eliminate the resistors, but a pair of capacitors is disposed across the lines and a conductor between the two condensers is grounded.
U.S. Pat. No. 2,277,216 (Epstein) discloses an electrical outlet unit which includes a filter system. The filter includes simply a capacitor connected to one of the conductors and grounded.
U.S. Pat. No. 2,621,252 (Pontius) discloses another type of filter system utilizing a coil and a ground capacitor.
U.S. Pat. No. 2,732,528 (Anderson) discloses a wave filter for suppressing alternating current interference while allowing dc signal pulses. The filter system includes two sections, a suppression-type section and a low-pass section.
U.S. Pat. No. 3,009,121 (Loebenstein) discloses an adjustable frequency rejection filter. The filter circuitry may be adjusted to reject selected alternating current components. The apparatus utilizes capacitors connected to ganged potentiometers. Circuitry for smoothing a non-linear circuit to remove noise from pulsed signals. The apparatus is designed particularly for smoothing pulsed modulated telemetry signals. A non-linear circuit is connected in series in one conductor, and a capacitor is connected between a pair of conductors. The non-linear circuit comprises a pair of diodes connected in parallel, back to back relationship.
U.S. Pat. No. 3,305,801 (Hartenstein) discloses a smoothing circuit which includes a non-linear circuit connected in series in one conductor of a two conductor pair, and a capacitor is connected across the two conductors. The non-linear circuit includes a pair of diodes connected in parallel in a back-to-back relationship. The circuit is designed to eliminate noise from pulse signals typically used in telemetry.
U.S. Pat. No. 3,402,372 (Wasyluk) discloses various RC networks built on monolithic circuit structures from telephone circuits used by modems and facsimile transmission equipment. Accordingly, none of the above-discussed filter systems is capable of filtering noise from such telephone lines. The apparatus of the present invention includes circuitry designed specifically for filtering undesirable noise from telephone lines used for modems and facsimile transmission equipment.